it would have been nice if BlackBerry had that as a disclaimer before I signed in.
The good thing is I only know one person who even uses BBM anyway

It was in a blog post from back in September. Basically, there was an exchange of custodianship for BBM IDs between Blackberry Inc and the company it contracted to service BBM on iOS, Android and all other platforms.

Blackberry Inc held on to responsibility for Enterprise BBM accounts, and for personal BBM accounts that were currently registered on BBOS/BB10 devices. The 3rd party developer took over responsibility for personal BBM that were registered on devices from all other platforms.

Early in the phase-in period, one-way migration was possible (from BBOS/BB10 to the company responsible for Android/iOS/others), but not the other way around.

Now, you can only perform lateral migration (ie, from one BBOS/BB10 device to another, or from one Android/iOS/others device to another). You can no longer make any platform changes that would require your BBM account to be transferred in either direction between Blackberry Inc and that other service provider.